In electrophotography, a latent image is created on the surface of an insulating photoconducting material by selectively exposing an area of this surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor surface, either exposed or unexposed to light, depending upon the relative electrostatic charge on the photoconductor surface and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and is then passed close the photoconductor surface, pulling the toner from that surface onto the paper or the transfer medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls melts and fixes the toner on the paper, subsequent to direct transfer (or indirect transfer where an intermediate transfer medium is used), producing the printed image.
Metal cores, often made from aluminum alloys, are typically used as substrates for organic photoconductors. They provide mechanical stability and serve as a base upon which the various functional layers may be coated, deposited or otherwise created. These aluminum cores are often anodized or coated with a subbing layer to provide adequate electrical stability and defect control in the final product. In most commercial, organic photoconductors, a charge generation layer (CGL) is coated upon this processed core. This layer is needed for light absorption and exciton creation. A charge transport layer (CTL) is typically coated over the CGL. This layer plays a role in exciton separation and charge transport. Typically, it is only the outer-most CTL that becomes worn or damaged during photoconductor use in a printer or copier. Therefore, if the CTL could be selectively removed and the remaining processed core and organic layers could be reintroduced into the photoreceptor coating process at the CTL-coating step, then a considerable savings in material cost and process time could be achieved. Significant environmental advantages would also attend such a process.
The potential importance of photoconductor recycling is best demonstrated by the number of patents that presently exist in this area. These patents generally teach the removal of all layers of the photoconductor down to the metal core; there is no selective removal of a specific layer. The majority of such photoconductor recycling techniques use abrasive methods to remove the various layers: brushes (see, for example, JP 11295908; Chem. Abs. 131: 315812), blasting polishing slurries (see, for example, JP 10239869; Chem. Abs. 129:267854), wiping (see, for example, JP 09211875; Chem. Abs. 127:212493), and jetted streams (see, for example, JP 09179326; Chem. Abs. 127:169041), are all known for use for this purpose. These methods suffer from a lack of selectivity in that all layers are removed down to the native metal core. Another method used for layer removal involves immersion of the photoconductor into a solvent system, wherein the organic layers swell and eventually dissolve away. This is a slow, time-consuming, non-selective process, and special or noxious chemicals must typically be added to accelerate the removal process (see, for example, JP 10213911, Chem. Abs. 129: 209296; and JP 08146623,Chem. Abs. 125: 181239). Harsh chemicals, such as strong acids, not only non-selectively remove all organic coatings, but also potentially damage or remove the anodization layer. Examples of such prior art processes follow.
U.S. Pat. No. 5,858,106, Ohmi, et al., issued Jan. 12, 1999, describes a method for removing organic films from semiconductors at room temperature. The key is that the films are peeled off rather than being dissolved which damages the semiconductor surface. In this process, the surface to be cleaned is placed in a solution comprising a mixture of organic solvents, such as isopropanol, water, and halogenated alkali salts (such a potassium chloride or potassium fluoride), and ultrasonic energy is applied.
U.S. Pat. No. 5,746,836, Fukai, issued May 5, 1998, describes a method for removing all photosensitive layers from a drum utilizing a mixture of water and a solvent (such as dimethyl succinate), and applying ultrasonic energy for a period of twenty minutes or more. The process removes all layers of the electrophotographic coating, not just the charge transport layer selectively.
U.S. Pat. No. 5,437,729, Boatner, et al., issued Aug. 1, 1995, describes a method for removing the top layer of a ceramic surface selectively by implanting ions into the surface at selected depths and locations, placing the surface in a liquid medium (such as water, isopropanol, butanol, hexane, or ether), and applying ultrasonic energy to the liquid medium. The use of this process on photoreceptors is not disclosed or suggested.
U.S. Pat. No. 5,403,627, Wilbert, et al., issued Apr. 4, 1995, describes a method for removing all layers of photoreceptor coating but only on a portion of the photoconductive drum. This is accomplished by directing a high intensity energy source (such as a laser, ultrasonic source, or heat) to a portion of the drum and using a gas or liquid jet to help remove the coating. The ultrasonic energy is applied through a liquid/solvent bath (such as methylene chloride and/or trichloroethylene).
U.S. Pat. No. 4,858,264, Reinhart, issued Aug. 22, 1989, describes a method for removing coatings (for example, protective coatings from aircraft) without requiring the use of solvents. The process uses mechanical energy of a reciprocating or vibratory nature to achieve the coating removal without damaging the underlying metal substrate.
U.S. Pat. No. 5,723,422, O'Dell, et al., issued Mar. 3, 1998, describes a process for cleaning photoreceptor substrates (for example, the removal of petroleum-based cutting oils) prior to applying the photosensitive layers. The cleaning solution utilized comprises a weak acid, borax or a polyphosphonate, an oil-soluble surfactant, and a water-soluble surfactant (such as polysorbate or a polyethylene/polypropylene copolymer). Ultrasonic energy may be applied to the part submersed in the cleaning solution.
U.S. Pat. No. 5,170,683, Kawada, et al., issued Dec. 15, 1992, describes a method for surface processing a drum used to make a photoconductor prior to application of the photosensitive layers. In this process, the surface of an aluminum roll is machined using a sintered polycrystal diamond with water as the cutting liquid. The photosensitive layers are applied after the drum surface has been treated.
U.S. Pat. No. 5,240,506, Liers, et al., issued Aug. 31, 1993, describes a method for cleaning ink residues from the surface of an engraved printing cylinder. In this process, the printing cylinder is rotated in a detergent solution while ultrasonic energy is applied to it.
U.S. Pat. No. 4,007,982, Stange, issued Feb. 15, 1977, describes a blade system for cleaning particulate matter off an electrophotographic imaging member during its use. The blade is vibrated parallel to the imaging surface using a high frequency vibration.
The challenge to achieving a selective CTL removal process is that it must be a process which would remove the CTL completely, while leaving other photoreceptor layers unharmed and undisturbed. Furthermore, since the population of used photoreceptor drums that would be introduced into the recycling process would, by its nature, include a wide range of CTL thicknesses, a varied binary removal process, i.e., one that in the time required to remove a thick CTL does not alter any other layers on a drum with a thin CTL, is necessary. For example, as the CTL is removed, the CGL cannot be removed or swollen, nor can the CGL components be extracted. Additionally, at least a portion of the electrical fatigue that builds up in a cycled photoreceptor has been thought to originate from processes occurring in the CGL. Hence, even if a process for selective removal of the CTL could be defined, it was not at all clear from the prior art that an electrically useful photoconductor could be produced just by recoating a new CTL. Surprisingly, we have found that upon placing a new or used photoreceptor drum (i.e., drums spanning the range of CTL thicknesses) into an ultrasonic bath containing certain specifically-defined solvents, the CTL coating may be quickly, easily and selectively removed, without damaging any of the other photoconductive coatings on the drum. Further, the electrical properties of reprocessed drums match those of drums that have passed through the photoconductor coating process just one time.